There is currently a trend in the automotive industry to replace combustion engines with electric motors or a combination of an electric motor and a combustion engine, thereby substantially reducing the environmental impact of automobiles by reducing (i.e., hybrids) or completely eliminating (i.e., electric vehicles) car emissions. This switch in drive train technology is not, however, without its technological hurdles as the use of an electric motor translates to the need for inexpensive batteries with high energy densities, long operating lifetimes, and operable in a wide range of conditions. Additionally, it is imperative that the battery pack of a vehicle pose no undue health threats, either during vehicle use or during periods of storage.
While current rechargeable battery technology is able to meet the demands of the automotive industry, the relatively unstable nature of the chemistries used in such batteries often leads to specialized handling and operating requirements. For example, rechargeable batteries such as lithium-ion cells tend to be more prone to thermal runaway than primary cells, thermal runaway occurring when the internal reaction rate increases to the point that more heat is being generated than can be withdrawn, leading to a further increase in both reaction rate and heat generation. Eventually the amount of generated heat is great enough to lead to the combustion of the battery as well as materials in proximity to the battery. Thermal runaway may be initiated by a short circuit within the cell, improper cell use, physical abuse, manufacturing defects, or exposure of the cell to extreme external temperatures. In the case of a battery pack used in an electric vehicle, a severe car crash may simultaneously send multiple cells within the battery pack into thermal runaway.
In order to prevent overheating due to an inadvertent short, thereby hopefully preventing the occurrence of a thermal runaway event, conventional battery packs typically position a fuse 101 in series with one, or both, interconnects that couple the battery pack 103 to the load 105 as shown in FIG. 1. Unfortunately, as fuse 101 is designed to prevent low resistance, high current shorts, it may not open and interrupt a sustained current of moderate magnitude such as that which may occur during an internal battery short. In these situations, the affected battery will continue to heat until a cell-level safety mechanism is activated.
Conventional cells, especially those prone to thermal runaway, typically incorporate a variety of different safety mechanisms into the cell itself as illustrated in the simplified cross-sectional view provided in FIG. 2. Battery 200 includes a cylindrical case 201, an electrode assembly 203, and a cap assembly 205. Case 201 is typically made of a metal, such as nickel-plated steel, that has been selected such that it will not react with the battery materials, e.g., the electrolyte, electrode assembly, etc. Typically cell casing 201 is fabricated in such a way that the bottom surface 202 is integrated into the case, resulting in a seamless lower cell casing. Electrode assembly 203 is comprised of an anode sheet, a cathode sheet and an interposed separator, wound around a center pin 204 to form a ‘jellyroll’. Typically center pin 204 is hollow, i.e., it includes a void running its entire length, thus providing a path for gases formed during an over-pressure event to escape the cell via the vent contained within electrode cap assembly 205. An anode electrode tab 207 connects the anode electrode of the wound electrode assembly to the negative terminal which, for an 18650 cell, is case 201. A cathode tab 209 connects the cathode electrode of the wound electrode assembly to the positive terminal via cap assembly 205. Typically battery 200 also includes a pair of insulators 211/213 located on either end of electrode assembly 203 to avoid short circuits between assembly 203 and case 201.
In cell 200, tab 209 is connected to cap assembly 205, which contains a current interrupt device (CID). The purpose of the CID is to break the electrical connection between the electrode assembly and the positive terminal 227 if the pressure within the cell exceeds a predetermined level. Typically such a state of over pressure is indicative of cell overcharging and/or of the cell temperature increasing beyond the intended operating range of the cell, for example due to an extremely high external temperature or due to a failure within the battery or charging system. Although other CID configurations are known, in the illustrated cell the CID is comprised of a lower member 215 and an upper member 216. Members 215 and 216 are electrically connected, for example via crimping along their periphery with a spot weld. Lower member 215 includes multiple openings 217, thus insuring that any pressure changes within case 201 are immediately transmitted to upper CID member 216. The central region of upper CID member 216 is scored (not visible in FIG. 2) so that when the pressure within the cell exceeds the predetermined level, the scored portion of member 216 breaks free, thereby disrupting the continuity between the electrode assembly 203 and the battery terminal.
Under normal pressure conditions, lower CID member 215 is coupled by a weld 219 to electrode tab 209 and upper CID member 216 is coupled by a weld 221 to safety vent 223. In addition to disrupting the electrical connection to the electrode assembly during an over pressure event, the CID in conjunction with safety vent 223 is designed to allow the gas to escape the cell in a somewhat controlled manner. Safety vent 223 may include scoring 225 to promote the vent rupturing in the event of over pressure.
The periphery of CID members 215/216 is electrically isolated from the periphery of safety vent 223 by an insulating gasket 226. As a consequence, the only electrical connection between CID members 215/216 and safety vent 223 is through weld 221.
Safety vent 223 is coupled to battery terminal 227 via a positive temperature coefficient (PTC) current limiting element 229. PTC 229 is designed such that its resistance becomes very high when the temperature exceeds a predetermined level, thereby limiting short circuit current flow. Cap assembly 205 further includes a second insulating gasket 231 that insulates the electrically conductive elements of the cap assembly from case 201. Cap assembly 205 is held in place within case 201 using crimped region 233.
In addition to the inclusion of a CID, a PTC and a safety pressure vent, many cells also utilize a separator within the electrode assembly 203 that is capable of impeding current flow once a predetermined temperature is reached. In such a separator, the material comprising the separator is designed to soften upon reaching the preset temperature (e.g., 130° C.), at which point the pores close and impede ion and current flow. Unfortunately if the temperature of the cell continues to increase, the material comprising the separator will completely melt and break-down, resulting in a massive internal short that can accelerate heating.
While individual cells may include one or more built-in safety mechanisms, as noted above, these safety mechanisms are not always effective when the cell is one of a large group of cells, i.e., the battery pack of FIG. 1. For example, the CID within a cell typically has a relatively low voltage rating and therefore may be subject to arcing and fire when it attempts to open in a high voltage battery pack. As a result, the cell may enter into a state of thermal runaway. Additionally, due to the excessive heat generated by such an event, the temperature of adjacent cells within the battery pack will also increase, potentially leading to a cascading effect where the initiation of thermal runaway within a single cell propagates throughout the entire battery pack. In such a situation, not only is power from the battery pack interrupted, but the system employing the battery pack is more likely to incur extensive collateral damage due to the scale of thermal runaway and the associated release of thermal energy. While it is possible to design a battery pack that minimizes the risks of arcing or excessive heating, such an approach leads to increased cost, complexity and weight, all of which may be quite significant in a large battery pack such as that employed in a hybrid or an all-electric vehicle. Accordingly, what is needed is a means of minimizing the risks associated with an internal short circuit within a battery pack, while not significantly impacting battery pack manufacturing cost, complexity and weight. The present system provides such a means.